In general, the methanol synthesis process predominantly relies on synthesis gas (syngas) as feed components. Syngas generally contains carbon monoxide and hydrogen. Carbon dioxide and nitrogen can also be present. Methanol production using syngas basically involves the following reactions:CO+2H2→CH3OH  (1)CO2+3H2→CH3OH+H2O  (2)
An additional side reaction includes:CO+H2OCO2+H2  (3)
Methanol synthesis is a strongly exothermic and equilibrium-limited reaction. Increases in reaction temperature tend to disfavor methanol formation, and tend to deactivate some of the more commonly used copper based catalysts. Thus, control of equilibrium in this complex reaction scheme is important to maximize the amount of methanol formed.
U.S. Pat. No. 4,968,722 discloses that two types of reactors are typically used for methanol synthesis: adiabatic bed reactor and cooled (i.e., isothermal) reactors. Adiabatic bed reactors typically have several fixed catalyst beds in series, and the temperature is controlled with heat exchangers between beds or by introducing cold synthesis gas between beds. Isothermal reactors typically have a bundle of tubes filled with catalyst, and use water to cool the tubes as the reaction takes place.
In the '722 patent, a particular reaction system is detailed in which the system uses multiple reactors in series. After each reactor, an absorption vessel is used to absorb methanol from the reactor effluent. Examples of absorbents used include tetra ethylene glycol dimethyl ether (TEGDME), sulfolane, and 18-crown-6.
U.S. Pat. No. 5,219,891 discloses a fluidized bed reactor that is used to make methanol. The reactor has catalyst in a plurality of interconnected fluidized bed sections, and each section is cooled by a heat exchanger. The temperature in the highest section is reduced to below the highest temperature in a lower section.
U.S. Pat. No. 5,449,696 discloses a process for production of methanol that uses a simulated moving bed. In the process, the carbon monoxide and hydrogen are catalytically reacted to form methanol. The methanol is separated from unreacted carbon monoxide and hydrogen by concurrent adsorption, using the carbon monoxide and hydrogen as the desorbent.
U.S. Pat. No. 6,723,886 discloses a process for production of methanol in a catalytic distillation unit, or CDU. This patent discloses that a number of stages can be used to achieve a final conversion of CO approaching 100%. The heat of reaction may be removed by removing a portion of the methanol, cooling it, and returning it to the CDU. It is not disclosed how the unreacted reactants are returned to the catalyst zone by distillation, since the syngas components are essentially non-condensable at methanol synthesis conditions.
A variety of reaction processes have been disclosed in an effort to find efficient ways of controlling equilibrium and/or temperatures in the complex methanol reaction process. Additional process schemes are still desired in order to maximize the conversion of CO, CO2 and hydrogen to methanol in much simpler and more effective ways.